There is a plug in the o2 spot on the new exhaust but the exhaust itself isnt fully connected, so technically yes the exhaust is open to atmosphere. When I replaced the cat I had to cut the bolts so its not bolted together its just held together by hangers till i get headers. For the most part the exhaust is together though, just a small crack between the 2 flanges. You can hear its open in the vids. Ill be working on it all today and tomorrow.